Many different types of machines, such as agricultural work vehicles, are usually steered by means of hydraulic steering systems having a continuously driven hydraulic pump that is connected via valve devices with a steering cylinder. The valve devices are hydraulically controlled by means of an operator interface (steering wheel) or sensors, for example, for the acquisition of plant rows in order to move the work vehicle on a desired path. To allow sufficiently rapid steering reactions, the conveyance volume of the continuously running hydraulic pump has to be dimensioned so that it is sufficiently large. This steering arrangement thus requires a relatively large amount of energy and contributes considerably to the losses in the powertrain of the work vehicle.
Moreover, in the case of forklifts, for example, so-called steer-by-wire steering systems are used, which by means of sensory registration of the turning position of an operator interface and of an electronic control device, adjust the turning angle of the steered wheels electromechanically, electrohydraulically or electropneumatically without mechanical connection between the operator interface and the steered wheels. Such a steering system may include a sensor that acquires the turning movements of the operator interface and transmits corresponding signals to a control device. Depending on the detected steering intention, the control device drives an electric motor whose output shaft is connected to a hydraulic pump, which in turn applies pressure to the chambers of the steering cylinder. Here, driving power for the electric motor is only needed if a turning movement is to be carried out.
In steer-by-wire steering systems, redundancy has to be provided to ensure functioning even if a component of the steering system fails. For this purpose, two electric motors and two hydraulic pumps are provided in a conventional embodiment, and the latter can be connected as desired by means of switching valves to the respective associated chamber of the steering cylinder. In the case of failure of an electric motor or of a pump, the steering cylinder is thus connected to the other pump that is still in running order. In another embodiment, the steering cylinder has four chambers, two of which are connected to one pump and the others of which are connected to the other pump. In this embodiment as well, only one of the two hydraulic circuits works at a time in each case.
Other conventional steer-by-wire steering devices from the automotive sector have two steering cylinders which are adjusted by associated electrohydraulic circuits, wherein, in normal operation, the two circuits are operated at the same time. In the case of failure, only one of the circuits is then still active for emergency operation.
In other conventional systems, it may be a disadvantage that the redundancy provided, within each case an electric motor and a hydraulic pump (and, in the embodiment with the four-chamber steering cylinder, also half of the steering cylinder), is without effect in normal operation. The steering speed and reaction is limited by the capacity of a single hydraulic circuit, which consequently has to be dimensioned to be sufficiently large so that a sufficient steering speed is achieved. The buyer thus acquires a number of expensive components and takes them along day after day in his/her vehicle although in most cases they are of no use to him/her, since they do not contribute to the steering action. This disadvantage may be avoided but at the expense of the use of two steering cylinders.
Therefore, the present disclosure provides an electrohydraulic steer-by-wire steering system which, in spite of redundancy, requires only one steering cylinder and exploits the capacity of the two hydraulic circuits in normal operation.